In order to provide enhanced communication services, 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service, named multimedia broadcast and multicast services (MBMS), has been proposed for introduction into release 9 (REL-9) of the Evolved Universal Terrestrial Radio Access (E-UTRA) standard, which is currently being defined. Examples of MBMS services and applications include multimedia broadcast, e.g. mobile television, audio, streamed video, etc. Some multimedia services require a high bandwidth due to the nature of the data content that is to be communicated, such as video streaming. Some multimedia services may only require a low bandwidth due to the nature of the data content that is to be communicated, such as news services. Typically, tens of channels carrying say, news, movies, sports, etc. may be broadcast simultaneously over a MBMS communication network.
Within the defined MBMS operation of Rel-9 of E-UTRA, a single radio transmission mode may be used, termed multicast broadcast single frequency network (MBSFN). In this point-to-multipoint (PTM) transmission mode of operation, multiple communication cells synchronously transmit the same MBMS content in their respective service areas. The area covered by the participating communicating cells of such a synchronised transmission is referred to as the ‘MSFSN’ area. Synchronous transmission of the same MBMS content is achieved by a central network entity, termed the Multi-cell/multicast Coordination Entity (MCE), which is configured to decide both the radio resources that are used for the MBSFN transmission as well as the details of the radio resource configuration, i.e. the layer-1/layer-2 (L1/L2) configuration parameters to be used.
A communication cell may participate in MBMS transmissions corresponding to different MBSFN areas, in which case MBSFN areas overlap. The radio transmission area of MBMS user data is the same as that used for the corresponding control information, i.e. the cells that participate in the transfer of the user data also participate in the transfer of the corresponding control information.
Within an MBMS service area, it is known that single frequency networks (SFN) may be employed. The allocation of subframes for MBSFN communication is complicated, involving several levels of communication protocol elements:
(i) Subframe Pool Reserved for Future use (SP-RF): This protocol element/field indicates sub-frames used in post release 8 systems, for new features such as MBSFN. The sub-frames indicated by the SP-RF, signalled via the Broadcast Control Channel (BCCH), are not relevant for REL-8 UEs. The SP-RF, specified by the field mbsfn-SubframeConfigList in SystemInformationBlockType2 (also referred to as SIB2), is defined by means of a list of Subframe Allocation Patterns (hereafter referred to as SIB2 SAPs).
(ii) MBSFN area specific Common Subframe Pool (CSP): This protocol element/field indicates which of the subframes indicated by the SP-RF are used for MBSFN. In case multiple MBSFN areas are used, the CSP needs to be defined for each of the MBSFN areas. These subframe allocations are referred to as the Common Subframe Pool (CSP), since they are common for all the multicast channels (MCHs) that are configured for a particular MBSFN area, i.e. each MCH uses a subset of the CSP.
(iii) MCH Subframe Allocation Pattern (MSAP): The allocation of radio resources to an (P)MCH is specified by means of an MCH Subframe Allocation Pattern (MSAP) i.e. the MSAP protocol element/field defines those subframes within a given periodic cycle that are allocated to a specific (physical) multicast transport channel ((P)MCH). At the time of filing this patent application, the details of the MSAP signalling have not yet been finalised.
(iv) MCH Dynamic Service Scheduling: This protocol element/field, which is provided per (P)MCH, indicates those subframes that are used for each of the services that are scheduled. E-UTRAN provides this information to the UE at the start of each scheduling period. The scheduling period is also referred to as an ‘MSAP occasion’. It should be noted that, within an MSAP occasion, all user data corresponding to an MBMS service is scheduled in ‘subsequent subframes’; i.e. subsequent when only considering the subset of subframes that are allocated to the concerned (P)MCH. Hence, for each service that is scheduled, E-UTRAN just needs to provide an indication of the start and the duration. At the time of filing this patent application, the details of the MSAP signalling have not yet been agreed. However, the duration of the MSAP occasion values that has been discussed is either 320 msec. or 640 msec.
MBMS technology is designed to transmit data traffic from a content server (often referred to as a data source) to multiple destination user terminals/user equipment (UEs) in a 3GPP cellular/mobile communication system. In order to achieve efficient transmission, two delivery modes have been defined for MBMS delivery in 3GPP mobile communication systems: point-to-multipoint (p-t-m) and point-to-point (p-t-p).
The decision of the delivery mode is made at a network controller, based on the number of UEs/users that have activated the particular MBMS service in the coverage area of the network controller. If the number of UEs that have activated the service is larger than a pre-set threshold value, p-t-m transmission is selected and used. Otherwise, the service is delivered in an uni-cast (i.e. point-to-point) manner, where a dedicated radio bearer to a particular UE is provided. This selection is made in order to optimise the efficiency of delivering the MBMS data content according to the number of participating users. In an uni-cast mode of operation, the E-UTRAN is aware of those services that each UE is receiving.
However, in contrast to the uni-cast mode, EUTRAN is generally not aware of the MBMS services that the UE is receiving. This implies that additional procedures need to be defined by which EUTRAN can determine or estimate the number of UEs that are interested to receive the MBMS service (sometimes referred to as a ‘counting procedure’). It has been agreed that the MBMS solution for release 9 will not include such kind of procedures. Consequently, release 9 only employs p-t-m transfer using MBSFN with a semi-static MBSFN area. Even if additional procedures are defined in later releases, p-t-m transfer mode is assumed to remain the typical and most cost-effective approach for the provision of multimedia services. However, with multicast/broadcast communications transmission/reception conflicts may exist, as described below.
In the field of 3GPP MBMS systems, the E-UTRA has been designed such that the E-UTRAN is able to configure a UE in a radio resource control (RRC) connected state, referred to as ‘RRC_CONNECTED’. In this state, the UE is configured to perform radio frequency measurements on:
(i) the frequency channel used by its serving communication cell,
(ii) other E-UTRA frequencies (referred to as inter-frequency measurements) and/or
(iii) frequencies used by other Radio Access Technologies (referred to as inter-RAT measurements).
Since UEs typically employ a single transceiver, it is not possible to perform inter-frequency and inter-RAT measurements, whilst the UE is engaged in (unicast) data transmission with the serving cell, as the transceiver is fully occupied in its uni-cast communications on a particular frequency channel. To overcome this, the E-UTRAN in 3GPP has been designed to configure periods in which it does not schedule any downlink data transfer. During these periodically appearing discontinuous reception (DRx) periods, also referred to as ‘measurement gaps’, the UE is able to perform the required measurements on other frequency channels, as detailed in (i)-(iii) above.
An example measurement gap configuration is illustrated in the timing diagram 100 of FIG. 1. Eight radio frames 105, 110, 115 up to 120 are shown. Each radio frame comprises ten sub-frames. As illustrated, a first subset 125 of radio frame ‘0’ 105, comprising six sub-frames is allocated for communicating prior to a ‘measurement gap’ 130 that comprises a gap of 6 msec. (i.e. 6 sub-frames). The current standard defines the repetition period of the ‘measurement gap’ as appearing either every 40 msec. or 80 msec. (i.e. every ‘4’ or ‘8’ radio frames), with an offset specifying the position within this gap period.
A perceived problem in the use of measurement gaps in an MBMS context, particularly when MBSFN is employed in a number of MBMS areas, is potential conflicts between the measurement gaps and MBMS service reception.
To clarify this, let us consider a scenario where two MBSFN areas are used, as shown in the timing diagram 200 of FIG. 2. A set of subframes 225 are allocated to a first MBSFN area that employs two transport channels, MCHa and MCHb. For the first MBSFN area, a first set of subframes 227 is allocated to the first transport channel MCHa comprising sub-frame ‘3’ 215 and sub-frame ‘8’ 220 in the first four radio frames 205 and a second set of subframes 232 is allocated to the second transport channel MCHb comprising a use of sub-frame ‘3’ 215 and sub-frame ‘8’ 220 in the next four radio frames. The remaining eight radio frames 230 in the sequence of sixteen radio frames are employed by the second MBSFN area MBSFN-2.
Let us also consider a scenario whereby five MBMS services are mapped to the first transport channel MCHa 227 and dynamic scheduling information is provided every thirty two radio frames.
Currently, within the MBMS standard, the MBMS services are scheduled according to a pre-defined scheme, which is configured semi-statically. More specifically, it has been agreed that the MBMS services are scheduled in the order in which they are listed on the multicast control channel (MCCH). This means that in every scheduling period, the first multicast transport channel MTCH-1 235 appears first, followed by the second multicast transport channel MTCH-2 240, and so on. This predefined order is beneficial for UEs that did not manage to receive the scheduling information.
For example, let us suppose that a first UE is only interested to receive the second multicast transport channel MTCH-2 240. If the first UE did not manage to receive the scheduling information from the MCCH, in principle the first UE then has to receive all subframes that are used for MCHa 227. However, when the UE detects a subframe in which MTCH-3 245 is scheduled, the first UE knows that any following subframes are not used for MTCH-2 240. Hence, the UE is able to stop reception until the next scheduling period. This feature is termed ‘early termination’. Furthermore, the UE is able to derive the MBMS service that the data corresponds to from the medium access control (MAC) header information (in particular from the logical channel identity).
Thus, systematic conflicts may exist between measurement gaps 130 of FIG. 1 and MBMS reception, for example in transport channels 215, 220. For example, a UE may be configured with a measurement gap 130 starting with sub-frame ‘6’ of the first radio frame of a set of eight radio frames, as illustrated in FIG. 1. If the same UE is interested to receive the MBMS service corresponding with MTCH-1 235 in FIG. 2, in every set of 32 radio frames, there is a conflict with one of the four measurement gaps that is configured for broadcast during this period, i.e. once every eight radio frames. If, however, the UE was interested to receive the MBMS service corresponding with MTCH-2 240 there would not be any conflict, as the MTCH-2 240 transmission does not conflict with the defined repetitions of the measurement gap. The same applies for MTCH-3 245, MTCH-5 255.
It is generally assumed that mobility measurements should take precedence over MBMS reception. Consequently, when there is a conflict between the two the UE will be unable to receive a part of the user data of the MBMS service. The impact of this depends on the service characteristics and the upper layer mechanism configured to overcome losses.